全文获取类型
收费全文 | 17066篇 |
免费 | 764篇 |
国内免费 | 618篇 |
专业分类
18448篇 |
出版年
2024年 | 27篇 |
2023年 | 344篇 |
2022年 | 217篇 |
2021年 | 314篇 |
2020年 | 395篇 |
2019年 | 624篇 |
2018年 | 568篇 |
2017年 | 492篇 |
2016年 | 458篇 |
2015年 | 548篇 |
2014年 | 947篇 |
2013年 | 1550篇 |
2012年 | 889篇 |
2011年 | 834篇 |
2010年 | 620篇 |
2009年 | 921篇 |
2008年 | 961篇 |
2007年 | 951篇 |
2006年 | 747篇 |
2005年 | 686篇 |
2004年 | 566篇 |
2003年 | 505篇 |
2002年 | 398篇 |
2001年 | 311篇 |
2000年 | 346篇 |
1999年 | 316篇 |
1998年 | 354篇 |
1997年 | 287篇 |
1996年 | 311篇 |
1995年 | 246篇 |
1994年 | 199篇 |
1993年 | 173篇 |
1992年 | 149篇 |
1991年 | 135篇 |
1990年 | 123篇 |
1989年 | 101篇 |
1988年 | 90篇 |
1987年 | 97篇 |
1986年 | 60篇 |
1985年 | 72篇 |
1984年 | 75篇 |
1983年 | 60篇 |
1982年 | 83篇 |
1981年 | 62篇 |
1980年 | 51篇 |
1979年 | 51篇 |
1978年 | 34篇 |
1977年 | 15篇 |
1976年 | 19篇 |
1972年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
小麦秆锈抗性遗传及抗性基因研究进展 总被引:1,自引:0,他引:1
目前国际上已发现近80个小麦抗秆锈基因,其中45个抗秆锈基因已被正式定名,58个抗秆锈基因已定位在小麦特定染色体上,其中12个基因被标记。本文对小麦抗秆锈病基因抗源、抗秆锈性遗传、分子标记研究现状及存在问题加以综述,并对抗秆锈分子遗传前景进行展望。 相似文献
82.
Diversity of an ectomycorrhizal fungal community studied by a root tip and total soil DNA approach 总被引:4,自引:0,他引:4
Molecular methods based on soil DNA extracts are increasingly being used to study the fungal diversity of ectomycorrhizal (EM) fungal communities in soil. Contrary to EM root tip identification, the use of molecular methods enables identification of extramatrical mycelia in soil. To compare fungal diversity as determined by root tip identification and mycelial identification, six soil samples were analysed. Root tips were extracted from the six samples and after amplification, the basidiomycete diversity on the root tips was analysed by denaturing gradient gel electrophoresis (DGGE). The soil from the six samples was sieved, total soil DNA was extracted and after amplification, the basidiomycete diversity in the soil fractions was analysed by DGGE. Fourteen different bands were excised from the DGGE gel and sequenced; fungal taxon names could be assigned to eight bands. Out of a total of 14 fungal taxa detected in soil, 11 fungal taxa were found on root tips, of which seven were EM fungal taxa. To examine whether the sieving treatment would affect EM species diversity, two different sieve mesh sizes were used and in addition, the organic soil fraction was analysed separately. DGGE analysis showed no differences in banding pattern for the different soil fractions. The organic fraction gave the highest DGGE band intensities. This work demonstrates that there is a high correspondence between basidiomycete diversity detected by molecular analysis of root tips and soil samples, irrespective of the soil fraction being analysed. 相似文献
83.
MH Daleke AD van der Woude AH Parret R Ummels AM de Groot D Watson SR Piersma CR Jiménez J Luirink W Bitter EN Houben 《The Journal of biological chemistry》2012,287(38):31939-31947
Mycobacteria use the dedicated type VII protein secretion systems ESX-1 and ESX-5 to secrete virulence factors across their highly hydrophobic cell envelope. The substrates of these systems include the large mycobacterial PE and PPE protein families, which are named after their characteristic Pro-Glu and Pro-Pro-Glu motifs. Pathogenic mycobacteria secrete large numbers of PE/PPE proteins via the major export pathway, ESX-5. In addition, a few PE/PPE proteins have been shown to be exported by ESX-1. It is not known how ESX-1 and ESX-5 recognize their cognate PE/PPE substrates. In this work, we investigated the function of the cytosolic protein EspG(5), which is essential for ESX-5-mediated secretion in Mycobacterium marinum, but for which the role in secretion is not known. By performing protein co-purifications, we show that EspG(5) interacts with several PPE proteins and a PE/PPE complex that is secreted by ESX-5, but not with the unrelated ESX-5 substrate EsxN or with PE/PPE proteins secreted by ESX-1. Conversely, the ESX-1 paralogue EspG(1) interacted with a PE/PPE couple secreted by ESX-1, but not with PE/PPE substrates of ESX-5. Furthermore, structural analysis of the complex formed by EspG(5) and PE/PPE indicates that these proteins interact in a 1:1:1 ratio. In conclusion, our study shows that EspG(5) and EspG(1) interact specifically with PE/PPE proteins that are secreted via their own ESX systems and suggests that EspG proteins are specific chaperones for the type VII pathway. 相似文献
84.
X. Pochon L. Garcia-Cuetos A. C. Baker E. Castella J. Pawlowski 《Coral reefs (Online)》2007,26(4):867-882
Recent molecular studies of symbiotic dinoflagellates (genus Symbiodinium) from a wide array of invertebrate hosts have revealed exceptional fine-scale symbiont diversity whose distribution among
hosts, regions and environments exhibits significant biogeographic, ecological and evolutionary patterns. Here, similar molecular
approaches using the internal transcribed spacer-2 (ITS-2) region were applied to investigate cryptic diversity in Symbiodinium inhabiting soritid foraminifera. Approximately 1,000 soritid specimens were collected and examined during a 12-month period
over a 40 m depth gradient from a single reef in Guam, Micronesia. Out of 61 ITS-2 types distinguished, 46 were novel. Most
types found are specific for soritid hosts, except for three types (C1, C15 and C19) that are common in metazoan hosts. The
distribution of these symbionts was compared with the phylotype of their foraminiferal hosts, based on soritid small subunit
ribosomal DNA sequences, and three new phylotypes of soritid hosts were identified based on these sequences. Phylogenetic
analyses of 645 host-symbiont pairings revealed that most Symbiodinium types associated specifically with a particular foraminiferal host genus or species, and that the genetic diversity of these
symbiont types was positively correlated with the genetic diversity found within each of the three host genera. Compared to
previous molecular studies of Symbiodinium from other locations worldwide, the diversity reported here is exceptional and suggests that Micronesian coral reefs are
home to a remarkably large Symbiodinium assemblage. 相似文献
85.
Inter simple sequence repeat (ISSR) marker assay was employed to validate the genetic fidelity of Swertia chirayita plantlets multiplied in vitro by axillary multiplication upto forty-two passages. Sixteen ISSR primers generated a total of 102 amplicons among the tissue-cultured
plants. Forty-eight amplicons were amplified in the outlier (a Swertia species). The outlier (negative control) was employed to rule out the possibility that the invariant fingerprint was due
to chance alone and that the ISSR technique employed was not discriminatory enough to detect the off-types. A homogenous amplification
profile was observed for all the micropropagated plants. The results confirmed the clonal fidelity of the tissue culture-raised
S. chirayita plantlets and corroborated the fact that axillary multiplication is the safest mode for multiplication of true to type plants. 相似文献
86.
M. T. Burrows 《Evolutionary ecology》1994,8(2):125-149
Summary A state-dependent model has been used to predict daily and tidal patterns of migration, feeding and inactivity in juvenile plaice (Pleuronectes platessa L.) in their intertidal and shallow subtidal nursery areas. Vertical position, quantity of energy reserves and fullness of the gut characterized the state of individual fishes. If feeding is visually mediated, the model predicts that, by night, plaice should move to areas of low predation risk and become inactive, whereas by day, plaice should migrate to feed in areas of high prey encounter rate. Vertical zones adopted by day and night and, hence, patterns of migration, should depend on the distributions of predators and prey. When prey are more abundant in the intertidal zone, plaice should move onshore to feed as the tide rises and when prey are more abundant offshore, plaice should move offshore to feed by day. If predators are equally abundant in all zones, the fish should behave as if no predators were present, having no effective refuge. An increase in the abundance of predators with depth results in the restriction of plaice activities to shallower vertical zones, depending on the magnitude of the predation threat. Zones adopted thus depend on the trade-off between energy intake and predation risk. Concordance between predicted behaviour and observed patterns is evident in contrasting habitats. Migration and feeding in the Wadden Sea, where prey are more abundant on intertidal flats, is dominated by the tidal component, whereas on impoverished exposed beaches of the west coast of Scotland, the diurnal component is dominant. Tidally related behaviour persists in the latter environment, not predicted by the model and may be a consequence of using endogenous rhythms to approach optimal behavioural patterns. 相似文献
87.
We describe an approach to analyzing single- and multiunit (ensemble) discharge patterns based on information-theoretic distance measures and on empirical theories derived from work in universal signal processing. In this approach, we quantify the difference between response patterns, whether time-varying or not, using information-theoretic distance measures. We apply these techniques to single- and multiple-unit processing of sound amplitude and sound location. These examples illustrate that neurons can simultaneously represent at least two kinds of information with different levels of fidelity. The fidelity can persist through a transient and a subsequent steady-state response, indicating that it is possible for an evolving neural code to represent information with constant fidelity. 相似文献
88.
Takuma Watanobe Naotaka Ishiguro Naohiko Okumura Masuo Nakano Akira Matsui Hitomi Hongo Hiroshi Ushiro 《Journal of molecular evolution》2001,52(3):281-289
The Kabukai A site (5 to 8C A.D.) of the Okhotsk cultural area is on Rebun Island, a small island near the coast, north–northwest
of Hokkaido, Japan. Specimens of Sus scrofa, called the Sakhalin pig, were discovered in five cultural layers at the Kabukai A site. Ancient DNA was extracted from the
remains of 42 Sakhalin pig bones. Thirty-nine nucleotide sequences of the 574-bp mitochondrial DNA control region, estimated
to have originated from at least 21 individuals, were amplified and analyzed phylogenetically. Nine distinct haplotypes (A1,
A2, A3, B1, B2, C1, C2, D1, and D2) from this site were classified into four haplotype groups (A, B, C, and D) by parsimonious
network analysis. Phylogenetic analysis of 9 ancient and 55 modern haplotypes indicated that the population of Sakhalin pigs
at the Kabukai A site belonged to two distinct clusters; haplotype groups A and B formed a cluster comprised only of themselves,
and haplotype groups C and D belonged to the cluster of one of the two genetic groups of Japanese wild boars uniquely distributed
in the western part of Japan, including one northeast Mongolian wild boar. Analysis of the haplotype distribution among three
archaeological sites and their historical transitions among the five layers reflecting the cultural periods at the Kabukai
A site suggests that the Sakhalin pig populations were introduced from Sakhalin island and the Amur River basin in the northeastern
Eurasian continent together with some cultural influences.
Received: 18 April 2000 / Accepted: 24 November 2000 相似文献
89.
《Saudi Journal of Biological Sciences》2022,29(5):3528-3538
Mentha longifolia is an important medicinal and aromatic perennial herb that exhibits wide distribution range from sub-tropical to temperate regions. In the present study, agro-morphological traits and genetic differences in 19 different populations of M. longifolia were studied to evaluate the level and extent of its diversity. Analysis of variance (ANOVA) showed that the different phenotypic characters show considerable differences among various populations and was significant at p < 0.05. Molecular diversity analysis performed by using arbitrary amplified eleven ISSR primers generated a total of 121 amplicons that range within the size of 200–2500 base pairs (bp). Each primer on average generated 11 amplicons with percentage polymorphism being 100. The analysis of molecular variance (AMOVA) showed more (64%) among population genetic diversity and less (36%) within the populations. Greater genetic differentiation (Gst = 0.6852) among these populations occurs due to low gene flow (Nm = 0.2297) and greater habitat variability. Geographic and genetic distances were positively correlated according to Mantel’s test. In order to remove any kind of biases, we used R software to perform cluster and redundancy analysis to analyse the extent of relatedness among studied populations. In terms of morphological and molecular aspects, the populations were grouped into four and five clusters respectively based on hierarchical clustering method. The results demonstrated that M. longifolia displays a great degree of morphological and genetic variation and can be utilized in breeding, genetic improvement, and gene bank conservation programmes in future. 相似文献
90.
The growth strategy of the Gram-positive rod 总被引:3,自引:1,他引:2
Abstract Bacteria grow by enlarging their envelope in such a way that osmotic pressure does not normally cause physical rupture. The strategy of Bacillus subtilis for both cylindrical elongation and pole formation is now substantially defined. Side-wall growth takes place by laying down new peptidoglycan, which is then displaced outwards, stretched and discarded; cross walls are laid down in the absence of stress, and then stretched and bulged outward as the septum is split and the pole is formed. 相似文献